1. Field of the Invention
The present invention relates to a technique for interpolating pixels in a spatial direction by interlace progressive transform, image enlargement and the like.
2. Description of the Related Art
As examples of processes for interpolating pixels in the spatial direction, there are interlace progressive transform (progressive scan conversion), image enlargement and the like. Of these, in the interlace progressive transform, generally an interpolation pixel value is calculated by weighting and adding an intra-field interpolation pixel value obtained by intra-field interpolation and an inter-field interpolation pixel value obtained by inter-field interpolation according to the motion of a pixel. Alternatively, either the intra-field interpolation pixel value or the inter-field interpolation pixel value is selected.
Since the pixels of each field exist only on every another line, generally bi-linear or bi-cubic interpolation in the vertical direction is performed using pixels on lines above or below a target interpolation pixel in the calculation of an intra-field interpolation pixel value. If pixel interpolation in the vertical direction is performed by these interpolation methods out of relation to the edge direction, aliasing called jaggies occur in inclined edge parts.
As a technique for suppressing the occurrence of jaggies by pixel interpolation in inclined edge parts, for example, Japan Patent Publication No. 2005-341346 discloses a technique for determining which the surrounding area of an interpolation pixel is, a complex pattern, a flat pattern, a vertical-line pattern, an angular pattern or the like by detecting a direction to interpolate using the differential value of pixel information between adjacent pixels in the horizontal and vertical directions in a field, and discriminating an inclined direction edge and the like from the complex pattern and the like.
FIG. 1 shows the interpolation method of the Japan Patent Publication No. 2005-341346. Each point in FIG. 1 represents a pixel and of these, an edge pixel is represented by black color. If it is determined that it is the above patterns in the above determination, pixel interpolation is performed in the vertical direction. If it is determined that it is other than the above patterns, pixel differences for each direction in the surrounding area of the interpolation pixel shown In FIG. 1 is calculated and pixel interpolation is performed determining a highly correlative direction as an interpolation direction, thereby to reduce jaggies.
As another technique for reducing jaggies, for example, Japan Patent Publication No. 2005-266918 discloses a technique for determining whether there is an inclined direction edge about a focused pixel and detecting the edge directions of a plurality of pixel positions corresponding to the focused pixel position including the focused pixel on the basis of the determination result.
Japan Patent Publication No. 2005-341337 discloses a technique for calculating the sum of the pixel information about an area around a focused pixel which is an interpolation target and its surrounding pixel areas respectively, detecting the highest correlative direction on the basis of the pixel information of each of the calculated areas and the positional relation between the areas and determining it as an interpolation direction.
Furthermore, as another technique, Japan Patent Publication No. 2007-67652 discloses a technique for determining an inclined direction of an edge in a position of a focused pixel, further determining a shape of the edge in the position of the focused pixel and executing appropriate interpolation of the edge in the inclined direction according to the result of the determination of the shape of the edge.
Furthermore, as another technique, Japan Patent Publication No. 2000-075865 discloses a technique for detecting an edge included in an image and calculating its direction, generating an adaptive sampling function by transforming a two-dimensional sampling function adapting it to the calculated edge direction and interpolating the edge part using the adaptive sampling function.
According to the technique disclosed by the Japan Patent Publication No. 2005-341346, the limit of a gently inclined interpolation direction is restricted by the width of a surrounding pixel area for determining the interpolation direction. Specifically, if the width of the surrounding pixel area is small, the gently inclined interpolation direction cannot be detected.
Conversely, if the width of a surrounding pixel area is large, even the gently inclined interpolation direction can be detected. However, if the width of a surrounding pixel area is large, it is often determined that the surrounding area is a complex area (pattern, etc.). Therefore, an edge direction for reducing jaggies is seldom interpolated, which is a problem.